Magnetoresistive elements feature an electrical resistance that strongly depends on the magnitude and/or a direction of an externally applied magnetic field. By means of magnetoresistive elements, electrical signals can be generated that are indicative of magnetic field strength and/or direction. These elements are therefore suitable for a large range of applications in the framework of magnetic field measuring and determination. In particular in applications for touch less measuring of rotation angles and revolution speeds, magnetic sensors making use of magnetoresistive elements play a predominant role. Also, magnetoresistive elements are widely applied for magnetic gradiometers as well as magnetic card reading and magnetic encoding devices.
There exists a large variety of different magnetoresistive elements exploiting various fundamental effects. For example, devices featuring an Anisotropic Magnetoresistive (AMR) effect, show a change in electrical resistance in the presence of a magnetic field. AMR sensors are typically made of a soft-magnetic material, such as nickel-iron (Permalloy), thin film deposited on e.g. a silicon wafer. Here, the magnetoresistive effect is mainly given by the relative direction between an electrical current flowing through the AMR element and the direction of magnetization of the soft-magnetic thin film.
Another effect denoted as Giant Magneto Resistance (GMR) can be exploited by making use of multilayer systems. Here, the magnetoresistive element features a stack of alternating magnetic and non-magnetic layers.
In an initial configuration, i.e. in the absence of an external magnetic field, magnetization direction of adjacent magnetic layers of the GMR element are coupled in an anti-parallel way. Variations of the electrical resistance arise due to modifications of the magnetization direction of the adjacently positioned magnetic layers. For instance, if a magnetic field is applied, the magnetic force between the magnetic field and the magnetization of the ferromagnetic layers attempts to align the magnetizations of the adjacent layers in a parallel way. Consequently, in the presence of a magnetic field, the magnetization orientation between two adjacent layers is something between parallel i.e. for strong magnetic fields, and anti-parallel, i.e. for zero or very low magnetic field, and depends on the strength of the magnetic field. Typically, the resistance of a GMR element is maximal for an anti-parallel orientation and becomes minimal for a parallel orientation of adjacent magnetic layers.
GMR-spinvalve systems typically feature two layers of ferromagnetic material and one layer of anti-ferromagnetic material. The antiferromagnetic layer is serving as a pinning layer that fixes permanently, i.e. pins, the magnetization direction of the first ferromagnetic layer along a predefined direction within the plane of the layer. The second ferromagnetic layer, also denoted as free layer can be magnetized in arbitrary directions in the plane of the layer and is free to follow the direction of an externally applied magnetic field. Mutual orientation of the magnetization of the pinning and the pinned layer finally determines the electrical resistance of the GMR-spinvalve system. Further, magnetoresistive elements may feature a number of similar effects, such as the Tunnel Magnetoresistive effects (TMR) that may exhibit when two adjacent ferromagnetic layers, e.g. of a GMR element, are separated by means of a thin layer of isolating material.
Magnetoresistive elements can be produced on a large scale by means of surface treatment and material deposition technologies that are, e.g. known in the field of semi-conductor manufacturing technology. For instance, hundreds or even thousands of magnetoresistive elements can be simultaneously produced on a common wafer in a cost efficient way.
Due to the symmetry of their underlying physical effects magnetoresistive multilayer elements inherently do not provide determination of the absolute direction of a magnetic field. However, by combining numerous magnetoresistive spinvalve elements, e.g. such as in a Wheatstone bridge, in principle also the direction of a magnetic field can be unequivocally determined. For instance, by making use of several GMR-spinvalve elements, each of which features a 180° ambiguity, directional magnetic sensors can be built, if the GMR-spinvalve elements and in particular their pinning layers are magnetized along different directions.
However, for various magnetic field sensor applications a manual positioning and orienting of various magnetoresistive elements is rather cost intensive and may reduce measurement accuracy of the magnetic sensor device. Also, magnetic sensors making use of magnetoresistive elements are commonly only sensitive to magnetic fields that coincide with the plane of the layered structures. Hence, magnetic sensors making use of magnetoresistive elements featuring a planar structure are typically insensitive to magnetic field components pointing in a direction that is substantially perpendicular to the planar surface of the magnetoresistive elements. Hence, building a magnetic sensor device capable of determining a magnetic field with respect to three spatial coordinates requires a rather sophisticated combination of magnetoresistive elements that are arranged in various orientations.
The present invention therefore aims to provide a magnetic field sensor device that is sensitive to magnetic field components that are perpendicular to the plane of a planar substrate.